Implantable devices come in a range of variants with varying geometrical sizes, materials and functions. Implantable devices include implantable medical devices (IMDs), such as cardiac pacemakers, cardiac defibrillators or cardioverters, implantable sensors, implantable catheters and implantable medical leads.
When implanting an implantable device a shell of water molecules will be created around the implanted material surface within nanoseconds. Shortly after this initial hydration layer, blood proteins and other macromolecules arrive at the surface. The protein adsorption process is dependent on the capability of the protein molecules to displace the tightly or loosely bound hydration layer. Depending on the interaction between the surface of the implantable device and the proteins the adsorption often causes conformational change of the proteins. This is of importance since the conformation changes can expose hidden epitopes, which may be responsible for initiating reactions such as inflammation, coagulation and foreign body response that eventually may lead to fibrous encapsulation. Also various cells are recruited and adhere to the surface of the implantable device, possibly via the adsorbed proteins. As a consequence, a thrombus is formed due to the accumulation of blood components including proteins and cells. The thrombus will over time start to fibrose and calcify into a tough fibrous capsule. This process causes the implantable device to adhere to the tissue, as for example the vascular wall. The tissue adherence will increase the risk of damaging the tissue when extracting the implanted device.
Thus, there is a general need to improve the surface characteristics of implantable devices to reduce protein adsorption and/or cell adherence and thereby reduce any risks associated with explanting the implantable device from a patient body.
Kolind et al., Biomaterials 31: 9182-9191 (2010) discloses the effect of surfaces structured with topographical features and their influence on cellular behavior. The article concludes that altering the interpillar gap size of the structures revealed a significant change in fibroblast proliferation, where larger interpillar gap sizes reduced fibroblast proliferation on Si-wafers coated with a surface layer of tantalum oxides.
Koh et al., Biomaterials 31: 1533-1545 (2010) investigates the influence of surface topography on fibrinogen and platelet adsorption on poly(lactic-co-glycolic-acid) films. The article concludes that low levels of fibrinogen adsorption and platelet response are achieved with surface structures having high aspect ratio with reduced interspacing or high density.
Milner et al., Journal of Biomedical Materials Research, Part A 76(3): 561-570 (2006) discloses platelet adhesion to polyether (urethane urea) surfaces textured with two different sizes of submicron pillars. The authors concluded that optimization of the pillar geometries could lead to a decrease in platelet adhesion by reducing the accessible surface area of the polyether (urethane urea) surface.
Kidambi et al., Tissue Engineering 13(8): 2105-2117 (2007) demonstrates that micro-topography of the surface of polydimethylsiloxane (PDMS) surfaces with poly(diallyldimethylammonium chloride)/sulfonated poly(styrene) coating influences cell adhesion and proliferation is dependent on pattern size and pitch.
Cortese et al., Langmuir 24(6): 2712-2718 (2008) discloses a hierarchical micro- and nanostructured polydimethylsiloxane superhydrophobic surface with tuned wettability.
US 2007/0148206 discloses a nanoengineered sculptured thin film of para-xylylene derivative polymer that can be used on implantable devices to prevent fibrous encapsulation and infections on the film. The document, though, also mentions that the film promotes cell differentiation, proliferation and adhesion.
The above described documents discuss various solutions related to the problem of protein adsorption and/or cell adhesion to surfaces. There is, though, still room for improvements within the technical field of implantable devices in terms of optimizing surface characteristics.